Electric field at a distance from a charged disk
A disk of radius 2.4 cm carries a uniform surface charge density of 3.1 μ C/m2. Using reasonable approximations, find the electric field on the axis at the following distances.
I have used the equation E=(Q/ε0)(1/(4*pi*r2))
I also tried the equation E=(Q/2(ε0))(1-(z/(√(z2)+(r2)))
Thanks in advance for the help. Both equations have not led me to the correct answer.
*note, there is not a figure provided for this question*